Instruments for respiratory gas analysis for medical diagnostics or lifestyle uses are increasingly common on the market. Particularly the development of inexpensive, selective, and highly sensitive sensors makes possible the development of small, portable, and inexpensive instruments in the future.
There are a multiplicity of biomarkers, metabolic products, and other substances which can be measured in respiratory gas and can provide information about inflammatory diseases, cancers, metabolic diseases, or poisoning symptoms of the patient; see, for example, Pleil in J Toxicol Environ Health B Crit. Rev. 2008 October; 11(8): 613-29. Role of exhaled breath biomarkers in environmental health science or Buszewski et al, Human exhaled air analytics: biomarkers of diseases, Biomed Chromatogr. 2007 June; 21(6): 553-66. Review. In practice, respiratory gas analysis is already used in the diagnosis of poisonings, asthma, diabetes, lung cancer, inflammatory respiratory diseases, and kidney or liver failure.
The analysis of respiratory gas can provide information about many metabolic processes of the human body. It is thus possible, for example, to infer the alcohol content of the blood, to diagnose an infection of the gastrointestinal tract with Heliobacter pylori bacteria, or to deduce the gestation cycle of a woman from the profile of the CO2 content.
To check the training efficiency of athletes, it can be helpful to measure the acetone content of the respiratory air. It is very helpful to monitor the NO content of the respiratory air to improve disease management of asthma patients.
In respiratory gas analysis, it is, however, important in most cases for the clinical value of the measured result that the correct portion of the volume exhaled is measured. The first portion of the air of an exhalation process comes from the oral and pharyngeal cavities and also the upper bronchi, the middle portion comes from the bronchi and bronchioles, the last portion (“end-expiratory”) in particular from the alveoli. Thus, for example, it is important for the diagnosis of the inflammatory reaction of asthma patients to measure the middle portion of the volume exhaled.
For an optimized measurement instrument for respiratory gas analysis, it is therefore necessary to have an apparatus and a method for controlling the patient's respiratory flow through the measurement instrument such that really the clinical important portion of the respiratory volume is conducted over the measurement sensor and, at the same time, the patient can breathe as naturally as possible. An unnatural breathing process (e.g., against too high an air resistance, or an abrupt termination of the expiration by the valve closure) would be, firstly, uncomfortable for the patient when using the instrument and could, secondly, distort the desired measured result. The technical solution for this depends crucially on whether the gas sensor has to operate in a continuous gas flow, or in a standing volume of a closed chamber.
Existing systems for NO analysis of respiratory gas measure in a continuous flow due to the widely used sensor principle. The routing of gas flow, which is used in these instruments, is less suitable for measurement in a sealed measurement chamber.